Field set are also useful when creating an integration: You can add and remove fields from the exposed API, without changes in the code, for example in the code below we have created a field set for the Account object named “IntegrationFields”:

DF13_SalesforceIntegrationUsingFieldSet.cls

//Salesforce Force.com Apex example: integration using field-set

@RestResource(urlMapping='/Account/*')
global with sharing class MyRestResource { 
    @HttpGet
    global static Account doGet() {
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        String accountId = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);
  String query = 'SELECT ';
                for(Schema.FieldSetMember f : SObjectType.Account.FieldSets.IntegrationFields.getFields()) 
{
                    query += f.getFieldPath() + ', ';
                }
                query += 'Id FROM Account where ID=\''+ id +'\' LIMIT 1';       
Account result = Database.query(query);
        return result;
    }   
}